1. Field of the Invention
This invention concerns the identification of objects by detecting retro-reflected light of a particular nature such as selected wavelengths or of a selected modulated form, and particularly identification by retro-reflecting an interrogating beam from a phase conjugate mirror.
2. Prior Art
Phase conjugate waves and methods and means of producing them are known. A phase conjugate wave generated by degenerate four wave mixing corresponds to a conjugate lightwave moving in the opposite direction, but otherwise identical in nature, to a second light wave with the phase of the conjugate wave reversed relative to the second wave. Phase conjugate waves can be viewed as a type of reflection of an incident, coherent wave combined with phase reversal. It is equivalent to leaving the spatial part of the electric field equation for a light wave unchanged and reversing the sign of the temporal term. The ratio of reflected power to incident power may be greater than unity (i.e., the system may have gain).
Striking consequences of phase conjugation include an incident beam being reflected from a "phase conjugate mirror" and directing the reflected beam exactly back along the path of the incident wave; and an incident beam which passes through an aberrator being distorted and reflected from a conjugate mirror, and passing back through the aberrator to emerge distortion free.
"Applications of Optical Phase Conjugation", C. R. Giuliano, Physics Today, pages 27 through 35, April 1981 describes the above basics of phase conjugation plus many applications of phase conjugation (the Giuliano article is incorporated herein by reference along with "Phase Conjugate Optics and Real-Time Holography", A. Yariv, IEEE Journal of Quantam Electronics, Vol. QE-14, No. 9, September 1978). One application disclosed is the use of phase conjugate waves in covert communications. Giuliano further discloses that four wave mixing (i.e., a nonlinear optical process where three input waves mix, often at a conjugate mirror, to yield a fourth or output wave), can be used to communicate information to one or more remotely located mobile receivers from an air or space-borne platform. The four wave mixer is situated in the transmitter and the receivers are equipped with interrogating lasers. At prearranged times, the interrogators illuminate the remotely located transmitter with lasers tuned to a predetermined operating frequency. With one of the three input waves (i.e., one of the pump waves) pulse modulated (i.e., temporal modulation), information can be transmitted back to the interrogation site as a modulated conjugate wave. Only those sites possessing the proper interrogating capability can obtain the information. Several other pump parameters, such as phase variations, can be imposed on the pump waves.
It would be advantageous to determine if a target object is a friend or foe without the need of modulation of the pump beams. It would further be advantageous to make such an identification in conditions as extreme as a battlefield where obscurants are present which are not normally present in the atmosphere. Finally, a more secure system of receiving identifying signals and information than that described by Giuliano is desirable.